Multi-speed bicycles which have several different gear ratios are very common, and the cyclist typically selects one of the gear ratios by operating a manual control. Manual shifting in this manner has several disadvantages, including the fact that it is a relatively slow way to change gears. Further, the cyclist must develop a great deal of skill in order to select the gear in which he or she can pedal most efficiently under the existing circumstances, and if the cyclist selects the wrong gear or fails to change gears as the cycling conditions change, the cyclist can become tired more rapidly than he or she should. Accordingly, attempts have been made to develop multi-speed transmissions for bicycles which automatically shift to a different gear when conditions warrant.
For example, U.S. Pat. Nos. 3,830,521, 4,435,997, 4,352,503, 3,926,020, 3,540,309, and 4,277,986, each disclose a transmission which is responsive to the speed of the vehicle for automatically varying the gear ratio being used. However, many of these transmissions are very complex and have parts which move continuously during normal use, thereby leading to frequent need for repairs. Moreover, a serious problem with transmissions of this type, which are responsive only to speed, is that under certain circumstances they make it very difficult for the cyclist to pedal. For example, if the bicycle is moving at a moderate to high speed and the transmission has therefore shifted to a higher gear, and if the cyclist then wishes to quickly accelerate or comes to a hill which must be ascended, it is necessary to slow the bicycle down in order to get the transmission to downshift to a gear which is more efficient for acceleration or climbing hills. Obviously, slowing of the speed of the bicycle makes no sense at all in either of these situations.
Above-mentioned U.S. Pat. No. 4,277,986 also discloses an alternative arrangement in which a bicycle transmission changes gears in response to the amount of force which the cyclist is applying to the pedals of the bicycle, or in other words the amount of rotational torque which the cyclist is attempting to apply to the rear wheel of the bicycle. Similar arrangements are disclosed in U.S. Pat. Nos. 3,969,948, 4,447,068, 3,995,508, 3,831,978, and Re. 30,758. Transmissions of this type are typically somewhat more practical than those which are responsive solely to the vehicle speed, but they are not completely satisfactory because they do not take into account the actual speed of the bicycle. Further, they tend to be relatively complex and to have many moving parts which move all the time, for example a plurality of small gears which are rotative continuously as the bicycle is pedaled, thereby making these arrangements subject to failure at an undesirably high rate.
It is therefore an object of the present invention to provide an automatic transmission for a manually propelled vehicle such as a bicycle which is simple and dependable and which will automatically and efficiently change gears in response to the speed of the vehicle and preferably also in response to the torque being manually applied by the operator.
A further object of the present invention is to provide a transmission, as aforesaid, which is relatively compact and has little or no relative movement between the parts thereof except at those points in time when the transmission is actually effecting a change in gearing ratios.
A further object of the present invention is to provide a transmission, as aforesaid, which is very rugged and requires minimal maintenance, and can be manufactured and assembled at a relatively low cost.